The photographic film industry in the United States and Canada consumes 1,555 tonnes and 466 tonnes, respectively of silver per year. About 35% of this silver is used for manufacturing x-ray film for which hospitals and medical clinics are the primary consumers. The manufacture and processing of x-ray film is described in the literature (The Manual of Photography, 1978) and is summarized in FIG. 1. In this flow sheet, the silver halide crystals are deposited on a base, usually comprised of polystyrene, polyester or polyethylene terephthalate. During the processing of x-ray film, the silver halide emulsion is exposed to radiation or light causing the reduction of a small fraction of silver halide to silver metal thereby forming a latent image of the subject. Subsequently, the film is treated with developing solution to activate the reduced nuclei of silver and promote the further reduction of silver halide until the latent image becomes visible. Finally, the developed film is treated with fixed solution to remove the unreduced silver halides.
The original x-ray film contains 7% to 8% Ag. During processing, about 80% of the silver is removed in the fixer solution and 20% remains with the film.
The recovery of silver from waste or spent solutions and materials has been studied by many researchers.
1. Aliotta, J. (1982) Silver Recovery and Refining from Solutions. In: M. I. El Guindy (Ed.), Precious Metals 1982, Proc. Sixth Internat. Prec. Metals Inst. Conf., Newport Beach, Calif., June 7-11, 1982, Pergamon Press, Toronto, pp. 597-601. PA0 2. American Hospital Association (1980), "Silver Recovery for Hospitals", 29 pp. PA0 3. Dannenberg, P. R. and J. M. Potter (1968). Silver Recovery from Waste Photographic Solutions by Metallic Displacement. U.S. Bur. Mines Report of Investigation 7117. PA0 4. Eastman Kodak Company (1976). Recovering Silver from Photographic Materials. Pamphlet p. 10. PA0 5. Kunda, W. (1981a) Treatment of Complex Silver Arsenide Concentrate in Nitric Acid System. In: R. O. McGachie and A. G. Bradley (Ed.), Precious Metals, Proc. Fourth Internat. Prec. Metals, Inst. Conf. Toronto, June, 1980, Pergamon Press, Toronto, pp. 39-57; Can. J. Chem Eng. PA0 6. Kunda, W. (1981b), Hydrometallurgical Process for Recovery of Silver from Silver Bearing Materials. Hydrometallurgy, 7, 77-97. PA0 7. Kunda, W. (1984a). Processing of Photographic Spent Solution by Chemical Method. In: D. A. Reese (Ed.), Precious Metals 1983, Proc. Seventh Internat. Prec. Metals Inst. Conf., San Francisco, June 12-16, 1983, Pergamon Press, Toronto, pp. 185-95. PA0 8. Kunda, W. and T. H. Etsell (1986). Recovery of Silver from X-ray Film. In: E. D. Zysk and J. A. Bonucci (Ed.), Precious Metals 1985, Proc. Ninth Internat. Prec. Metals Inst. Conf., New York, June 10-13, 1985, IPMI Press, Allentown, Pa. pp. 289-304. PA0 9. Kunda, W. (1984b). Hydrometallurgical Processing of Silver Concentrate. In: V. Kudryk, D. A. Corrigan and W. W. Liang (Ed)., Precious Metals: Mining, Extraction and Processing, Met. Soc. AIME, Warrendale, Pa., pp. 397-423. PA0 10. Kunda, W., B. Rudyk and H. Veltman, Recovery of Elemental Sulphur from Sulphur Bearing Materials. Presented at Can. Sulphur Symp., Calgary, May 30-31, 1974, 12 pp. PA0 11. The Manual of Photography (1978), 7th ed., Focal Press, London.
59, 347-56.
These processes can be grouped in the follow categories: cementation, electrowinning and chemical precipitation. According to the American Hospital Association Report, the electrowinning method is almost exclusively used for silver recovery from spent photographic fixer solutions. In spite of the popularity of this process, the electrowinning method has many disadvantages; i.e., cost of electricity, high capital investment, labor costs and inadequate silver recovery.
More efficient silver recovery can be obtained by chemical precipitation with hydrogen sulfide as disclosed in Kunda et al above. However, it requires a skilled operator to handle the toxic H.sub.2 S gas and has not been used commercially. Another process, which involves the use of hydrogen sulfide to precipitate silver is disclosed in U.S. Pat. No. 4,127,639. The patent relates to precipitating silver sulfide from aqueous lead solutions obtained by hot chloride leaching of residues containing lead and silver ore. The silver can be recovered from the silver sulfide.
It has been known for some time to use sodium sulfide to precipitate silver sulfide from the thiosulfate fixing solutions, as disclosed in U.S. Pat. No. 1,446,405, the sodium sulfide being less hazardous to handle. An improvement on this process is disclosed in U.S. Pat. No. 3,832,453 which discloses the use of sodium sulfide in a static mixer device which passes the mixture at high velocity to precipitate silver sulfide particles from the fixer solution.
It has been subsequently discovered, as disclosed in U.S. Pat. No. 4,437,889, that nitric acid may be added to photographic waste to precipitate from the waste solutions a residue which may be processed by heating and calcination to yield pure silver. The photographic fixer solution is heated to a temperature in the range of 45.degree. to 65.degree. C. With agitation, nitric acid is added at a 65% concentration and having a specific density of 1.4 grams per milliliter. A heavy loading of nitric acid is used which is considerably in excess of the amount of photographic liquid waste. With continued heating, the mixture is allowed to stand for digestion and the precipitate filtered from the mixture. The residue is then heated to a temperature in the range of 600.degree. to 700.degree. C. and then further fused in a graphite crucible at temperatures of 900.degree. to 1000.degree. C. to yield silver. This process for isolating silver from fixer solutions is energy intensive and requires large quantities of nitric acid. Furthermore, the recovered silver has a purity in the range of 99.5% and hence has to be further refined before it can be reused, since desired silver purity is usually in excess of 99.9%.
There is, therefore, a need for a safe, simple and inexpensive silver recovery process that can be used by small hospitals, photoprocessing labs in rural areas and like facilities.